'''
题目：有一分数序列：2/1，3/2，5/3，8/5，13/8，21/13...求出这个数列的前20项之和。

程序分析：请抓住分子与分母的变化规律。

程序源代码：


'''

# !/usr/bin/python
# -*- coding: UTF-8 -*-

a = 2.0
b = 1.0
s = 0
for n in range(1, 21):
    s += a / b
    t = a
    a = a + b
    b = t
print(s)

# !/usr/bin/python
# -*- coding: UTF-8 -*-

a = 2.0
b = 1.0
s = 0.0
for n in range(1, 21):
    s += a / b
    b, a = a, a + b
print(s)

s = 0.0
for n in range(1, 21):
    s += a / b
    b, a = a, a + b
print(s)

# !/usr/bin/python

from functools import reduce

a = 2.0
b = 1.0
l = []
l.append(a / b)
for n in range(1, 20):
    b, a = a, a + b
    l.append(a / b)
print(reduce(lambda x, y: x + y, l))
'''



'''









